Because of its excellent properties in chemical resistance, heat resistance, non-stickiness, and the like, fluoropolymers are used as preferred coating materials for different surfaces, including notably metal surfaces, for example, in applications which include linings for chemical units, which are required to be corrosion resistant; coatings for rice cookers, and cooking utensils that are required to be corrosion resistant and non-sticky.
However, the excellent non-stickiness inherent properties of fluoropolymer also often results in insufficient adhesion to substrate surfaces, in particular to metal surfaces, and a variety of solutions have been developed up to now for improving such adhesion.
More particularly, thermoplastic fluoropolymers which are known for possessing film-forming properties, such as tetrafluoroethylene/perfluoroalkyl vinyl ether copolymers (PFA), tetrafluoroethylene/hexafluoropropylene copolymers (FEP), ethylene/tetrafluoroethylene (ETFE) and ethylene/chlorotrifluoroethylene (ECTFE) polymers, and the like, are capable of exhibiting fluidity at or above their melting points and of adhering to metals with an adhesion strength too weak to be of any practical use. Thus, the conventional approach has been to chemically or physically roughen the metal surface, followed by a thermal fusion or adhering with the intermediary of an adhesive layer (also called primer) between the outmost fluoropolymer layer and the metal, which also has to possess outstanding adhesion properties towards additional top-coat (outer) layers made from fluoropolymers.
For conferring appropriate adhesive properties, primer compositions which have been suggested in the past, typically comprise certain aromatic polymers, including notably polyamideimide (PAI), polyimides (PI), sulfone polymer (SP) and the like.
In order to ensure optimized mixing between these components, it is current practice mixing the fluoropolymer and the aromatic polymer in an organic solvent, typically in N-methylpyrrolidone (NMP), possibly in admixture with other solvents.
Thus, EP 0789728 A (E.I. DUPONT DE NEMOURS) 2 Sep. 1998 discloses a primer composition comprising notably a polyether sulfone, a polyamideimide and/or a polyimide, a fluororesin and an organic solvent. N-methylpyrrolidone and its mixtures are the sole solvents which are mentioned and exemplified.
Similarly, U.S. Pat. No. 5,626,907 (E.I. DUPONT DE NEMOURS) 6 May 1997 discloses a primer composition comprising notably a fluororesin, a polyether sulfone polymer, at least one polymer selected from a polyimide and a polyamideimide, solubilized or dispersed in an organic solvent. This document teaches that the organic solvents which can be used include N-methylpyrrolidone either alone or in admixture with other solvents, like diacetone alcohol or xylene.
Nevertheless, a continuous need exists for primer composition possessing improved adhesion after prolonged exposure to harsh conditions (notably exposure to water vapour).
The present invention thus provides a solution for improving adhesion behaviour of fluoropolymer compositions to metal surfaces.